Circuit for adjusting the background brightness in television reproducing device



Jan. 2, 1962 T POORTER ET AL CIRCUIT FOR ADJUSTING THE BACKGROUND BRIGHTNESS IN TELEVISION REPRODUCING DEVICE Filed March 2, 1959 NVENTORS TEUN S POORT JOSUE JEAN PH LIPPE VA TON AGENT rates This invention relates to circuit arrangements for adjusting the background brightness in television reproducing devices of the type having at least two electron guns.

Such reproducing devices are used, for example, in colour television. Three electron guns are then usually provided which may be incorporated in separate cathoderay picture tubes but it is, of course, also possible for all three guns to be housed in a single tube.

Now as is well-known, in reproducing a colour television picture, it is desirable that a variation in the background brightness of a colour picture should be such that variation in colour does not occur. In view of the fact that the relationships between the variation in brightness brought about by the various guns and the voltage variations on the control electrodes of the relevant guns, each relating to a given basic colour of the scene to be reproduced, are generally different, it is not possible to bring about variation in background brightness without variation in colour by increasing or reducing the biassing potentials of the electrode systems of the three guns by the same amount.

The luminous flux 5, produced by a reproducing system, may be expressed substantially by:

wherein V represents the voltage between the cathode and the control grid of the associated electron gun, V is that value of this voltage for which is exactly zero, and C and 'y are constants which are dependent upon the structure of the tube and the kind of the phosphor used.

For the reproduction of white by means of three sys tems of which one contains the red basic colour, the second contains the green and the third contains the blue, it must be true that:

,: =a:b:c

where a, b and c are constants, the values of which are dependent upon the colour points of the phosphors chosen and upon the kind of the white to be reproduced. The

indices indicate the colour of the associated luminous flux.

From this follows for the voltages:

a /1 b 1/1 c 1/1 (a) (a) (a) wherein C C and C represent the values of C in the reproducing device, which supplies the red, the green or the blue luminous flux respectively. The values of 'y are usually substantially the same for the various colours.

This relationship must apply to all brightness values of the white to be reproduced.

An object of the invention is to provide a circuit arrangement in which it is possible for the biassing potentials of the various guns to be varied simultaneously so as to invariably satisfy the above-mentioned formula.

For this' purpose, the circuit arrangement according to the invention is characterized in that the biassing potential for' each electron gun is derived via a resistor, the voltage of which at a given point and the voltage drop across it being variable independently of each other, and that means are provided by which the voltage drop be- 3,015,751 Patented Jan. 2, 1962 tween the point on the resistor from which the biasing potential is derived, and the said given point, is increased or decreased by the same fraction for the various resistors.

In order that the invention may be readily carried into effect, three embodiments will now be described in detail, by way of example, with reference to the accompanying drawing, in which:

FIG. 1 is a circuit diagram of one embodiment of a background brightness arrangement according to the invention;

FIG. 2 is a circuit diagram of another embodiment of a background brightness arrangement according to the invention; and

FIG. 3 is a circuit diagram of still another embodiment of a background brightness arrangement according to the invention.

In FIG. 1, reference numerals 1,, 1 and 1,, indicate variable resistors connected in series with voltage sources V,., V and V and current sources 1,, l I respectively. Terminals 2 2 and 2 of the said resistors are each connected to an electrode of an electron gun, the electron guns each producing a luminous flux in a given basic colour.

The current sources and the voltage sources are all variable. The voltage drop across the resistors 1,, 1 1,, is dependent only upon the current supplied by the rel evant current source, since this current is independent of the relevant voltage source. However, the voltage of the terminals 3,, 3 3 of the resistors is dependent only upon the voltage supplied by the relevant voltage source, since this voltage is independent of the current flowing through the source.

The adjusted values of the variable resistors 1 determine what voltage drop across the relevant resistor, as a superposition on the voltage supplied by the relevant voltage source, is active at the associated electrode.

The individual brightnesses of the luminous fluxes produced by the various electron guns are controlled and hence the differences in the cut-off voltages V may be neutralized by control of the voltage sources V V and V By adjustment of the values of resistors 1, it is possible for the differences in the mutual conductances of the three electron-gun systems to be compensated for the general brightness control, and hence for the ratio between these voltage drops, with equal currents of the current sources 1,, I and 1 to be chosen equal to the desired ratio, which thus is the ratio between a /7 b 1/7 c /7 (0.) Y (a) end (a) If the background brightness of the picture to be reproduced is to be varied without colour variations in the image occurring, the ratio between the various voltage drops across the resistors 1, active at the electrodes of the guns, must not vary.

In the circuit arrangement shown in FIG. 1 this is achieved by maintaining the currents supplied by the cur: rent sources at equal values. Thus, by varying the three current sources simultaneously and in the same manner, the background brightness of the image to be reproduced is varied without variation in the colour of the image. This relationship between the three current sources is represented by the dashed line.

A desirable property of the circuit arrangement is that, if the currents supplied by the current sources are nil, the: voltages on the electrodes are equal to the voltages of the voltage sources, independent of the values of the resistors 1, so that the voltage sources may readily be adjusted to the proper values. If, now the current sources are caused .to supply current the values of the resistors 1 can also be readily adjusted.

It is to be noted that it is alternatively possible to compensate the differences in mutual conductance by adjustment of the current sources and to adjust the background brightness by suitable variation of the resistors ll. If the resistors 1 have the same values, this variation may be effected by varying the three resistors in the same manner. The desired ratio between the voltage drops across said resistors now adjusted by the current sources is not varied by this variation.

It is also to be noted that the variable resistors of FIG. 1 may, without objection, be replaced by fixed resistors in which the biassing potentials are derived from variable tappings, for example sliding contacts. The voltage drop between a terminal 3 and the electrode of a gun is, in this case, determined by the position of a tapping instead of by the adjusted value of a resistor.

FIG. 2 also shows a circuit arrangement according to the invention.

In this circuit arrangement, the resistors 4,, 4 and 4 are those, the voltages of which at a given point and the voltage drops across them are variable independently of one another. Said resistors, comprising sliding contacts 5,, 5 and 5 which are connected to the electrodes of the electron guns, are coupled via sliding contacts 6,, 5 and 6 to resistors 7,, 7 and 7 which are bridged by stabilising devices 8,, 8 and 8,, respectively. The resistors 4 are assumed to be large with respect to the resistors 7. In series with said stabilisers, resistors 9,, 9 9,, and 10,, 10,, 1% are connected between the terminals of a (common) voltage source V. In FIG. 2, the resistors 9 are assumed to be variable; as a result of this variation, the voltage on the junction point between a resistor 9 and a stabiliser 8 is varied. The adjustment of the sliding contacts 6 determines what parts of the voltage drops across the resistors 7, maintained constant by the stabilisers, are active across the resistors 4, the adjustment of the sliding contacts 5 determining what parts of the variable voltage drops are active at the electrodes of the electron guns.

The adjustment of the resistors 9 determines the individual brightnesses of the luminous fluxes produced by the various electron guns; the differences in cut off voltages V can thus be compensated by controlling the resistors 9. By adjustment of the positions of the sliding contacts 5, the differences in the mutual conductances of the three electron-gun systems are compensated for the general brightness control.

If the background brightness is to be controlled without colour variation of the image reproduced, the adjusted fractions of the voltage drops across the resistors 4 must invariably exhibit the desired ratio with respect to one another. If the voltages adjusted by the stabilisers are equal to one another, the sliding contacts 6 must occupy corresponding positions on the resistors 7. The coupling between the sliding contacts 6 which serves for this purpose is shown diagrammatically by a single dashed line. In practice, it is possible, for example, to utilise for this purpose three relatively equal potentiometers, the sliding contacts of which are moved by a common shaft.

In FIG. 2, the resistors 9 are assumed to be variable for the control of the voltage at the junction point between a resistor 9 and a stabiliser 8. However, this voltage may alternatively be varied by variation of the voltage across the series-combination of a resistor 9, a stabiliser 8 (with the associated combination of resistors 4 and 7), and a resistor 10. In this case, the various circuit arrangements are preferably provided with separate variable voltage sources. The resistors 9 may in this case be fixed resistors.

FIG. 3, at last, shows one embodiment of a circuit arrangement according to the invention in which voltage stabilisers are not required.

In FIG. 3, reference numerals 11,, 11 and 11 indicate the resistors having their sliding contacts 12 connected to the electrodes of the electron guns which each supply a luminous flux having a given basic colour. Said resistors are connected via sliding contacts 18,, 18 18 in series with adjustable portions of resistors 13,, 13 and 13 As before the resistors ll are assumed to be large with respect to the resistors 13. The last-mentioned resistors, which are relatively equal, as well as their adjustable parts, are coupled via resistors 14,, 14,,, 14 and 15,, 15,, 15 respectively, to which they are connected via sliding contacts 16,, 16,, 16 and 17,, 17,,, 17 respectively, to the terminals of a voltage source V (common to all networks).

The sliding contacts 16, and 17, may be moved along the resistors 14, and 15, so that a decrease or increase of resistor 14, results in an equal increase or decrease of resistor 15,. The coupling between the sliding contacts 1%, and 17, which serves for this purpose is indicated diagrammatically by a double dashed line. This implies that the voltage drop across the resistor 13, and also that across the adjustable part thereof and hence also the voltage drop across the resistor 11, are independent of the positions of the sliding contacts 16, and 17,.

Similarly thermovements of the sliding contacts 16 17 and 15 17,, respectively are such that the voltage drops across the resistors 13, and 13 are likewise independent of the positions of said sliding contacts.

The sliding contacts 1%,, 18,, 18 are connected together (shown diagrammatically by the single dashed line) so that the adjustable parts of the resistors 13,, 13 13,, are always substantially equal.

By means of the sliding contacts 12,, 12 and 12 it is possible to adjust the parts of the voltage drops across the resistors 11,, 11,, 11 which are supplied to the electrodes of the associated electron guns.

If the sliding contacts 12,, 12,, 12 are properly adjusted, as well as the positions of the sliding contacts 16 and 17, the background brightness may be varied by simultaneous variation of the positions of the sliding contacts 18 without the colour of the reproduced image varymg.

The circuit arrangements of FIGS. 2 and 3 naturally have the same properties with regard to adjustment as has the circuit arrangement of FIG. 1.

It is to be noted that, in FIG. 2, it is possible to interchange the potentiometer circuits constituted by the resistors 7 with their sliding contacts 6 and those constituted by the resistors 4 with their sliding contacts 5 and, in FIG. 3, the potentiometer circuits constituted by the resistors 13 with their sliding contacts 10 and those constituted by the resistors 11 with their sliding contacts 12. In this case, the compensation of mutual conductance is brought about by adjusting the sliding contacts 6 and 18 respectively and the background brightness is adjusted by means of the sliding contacts 5 and 12, respectively.

What is claimed is:

1. A circuit for adjusting the background brightness of an image reproducing device having at least two electron guns, comprising separate means for deriving a bias potential for each of said electron guns and means applying said bias potentials to the respective electron guns, each of said bias deriving means comprising a resistor having two terminals, means providing a first variable potential at one of said terminals with respect to a reference potential, means for varying the potential drop between said terminals independently of the potential at said one terminal, means applying the sum of said first potential and at least a portion of said potential drop to the respective electron gun, and means simultaneously adjusting said portions of the potential drops of said resistors by the same fraction with respect to the voltage between the terminals of the respective resistors.

2. A circuit for adjusting the background brightness of an image reproducing device having at least two electron guns, comprising separate means for deriving a bias potential for each of said electron guns and means applying said bias potentials to the respective electron guns, each of said bias deriving means comprising a resistor having two terminals and a variable tap, means applying a variable potential to one of said terminals with respect to a reference potential, means for varying the potential drop between said terminals independently of said variable potential, means connecting said variable tap to the respective electron gun, and means for simultaneously varying the variable taps of the resistors in each of said bias deriving means by the same fraction with respect to the terminals of the respective resistor.

3. A circuit for adjusting the background brightness of an image reproducing device having at least two electron guns, comprising separate means for deriving a bias potential for each of said electron guns and means applying said bias potentials to the respective electron guns, each of said bias deriving means comprising a resistor having two terminals, a variable voltage source connected between one terminal and a reference potential, a variable current source connected to the other terminal for varying the voltage drop between said terminals independently of the voltage at said one terminal, said resistor being variable, means applying the voltage at said other terminal to the respective electron gun, and means varying the resistors of the bias deriving means in a constant relationship.

4. A circuit for adjusting the background brightness of an image reproducing device having at least two electron guns, comprising separate means for deriving a bias potential for each of said electron guns and means applying said bias potentials to the respective electron guns, each of said bias deriving means comprising a resistor having two terminals, means providing a variable potential at one terminal with respect to a reference potential, means for varying the current flowing through said resistor independently of the potential at said one terminal, means applying the potential at the other terminal to the respective electron gun, and means simultaneously varying the potentials at said second terminals of each resistor in the same rela tionship to the currents flowing through the respective resistors and independently of said variable potentials.

5. A circuit for adjusting the background brightness of an image producing device having at least two electron guns, comprising separate means for deriving a bias potential for each of said electron guns, and means applying said bias potentials to the respective electron guns, said bias deriving means each comprising a first resistor having first and second terminals, means providing a variable voltage at said first terminals with respect to a reference potential, a second resistor having one end connected to said first terminal, means providing a constant voltage drop across said second resistor, means connecting said second terminal to a variable tap on said second resistor, variable tap means on each first resistor connected to the respective electron gun, and means simultaneously varying the voltages between said first terminals and the variable taps on said first resistors in a constant ratio.

6. The circuit of claim 5 comprising a voltage regulating device connected in parallel with each second resistor, and a source of voltage and resistance means connected in series with said device.

7. The circuit of claim 6 in which said resistance means comprises a variable resistor.

8. The circuit of claim 5 comprising a voltage source, and first and second resistors simultaneously variable in opposite directions connected in series between opposite ends of said second resistor and said voltage source.

9. A circuit for adjusting the background brightness of an image producing device having at least two electron guns, comprising separate means for deriving a bias potential for each of said electron guns, and means applying said bias potentials to the respective electron guns, said bias deriving means each comprising a first resistor having first and second terminals, means providing a variable voltage at said first terminals with respect to a reference potential, a second resistor having one end connected to said first terminal, means providing a constant voltage drop across said second resistor, means connecting said second terminal to a variable tap on said second resistor, variable tap means on each first resistor connected to the respective electron gun, and means simultaneously varying the taps of all of said second resistors the same fraction with respect to the total resistance of the respective second resistor.

10. A circuit for adjusting the background brightness of a television image producing device having at least two electron guns, comprising a voltage source and separate means for deriving a bias potential for each of said electron guns, each bias deriving means comprising, first, second and third resistors serially connected to said source, means for varying the potential at one end of said second resistor without varying the voltage drop across said second resistor, a fourth resistor, means connecting said fourth resistor between said one end of said second resistor and a variable tap on said second resistor, a variable tap on said fourth resistor connected to the respective electron gun, and means simultaneously varying the voltages between said one end of said second resistor and the variable tap on said fourth resistor of each bias deriving means in a constant ratio.

References Cited in the file of this patent UNITED STATES PATENTS 2,371,897 Knick Mar. 20, 1946 2,693,550 Doherty Nov. 2, 1954 2,884,562 Solow Apr. 28, 1959 2,888,604 Ralston May 26, 1959 2,906,917 Simpson Sept. 29, 1959 

